1. Field of the Invention
This invention relates to a polyimide resin for melt forming and more particularly, to a process for preparing a polyimide which has good thermal stability and good forming processability.
2. Description of the Prior Art
Polyimides obtained by reaction between tetracarboxylic dianhydrides and diamines have been heretofore used in the fields of electric and electronic apparatus, aerospace apparatus and transportation apparatus since they have, aside from high heat resistance, good mechanical strength and dimensional stability along with inflammability and electric insulating properties. They are expected to be widely employed in the fields requiring a high heat resistance.
However, some polyimides have a good heat resistance, but do not have a distinct glass transition point, so that when applied as a forming material, they have to be processed by the use of a sinter molding technique. Alternatively, some polyimides exhibit good processability but have a low glass transition temperature and are soluble in halogenated hydrocarbons, thus being not satisfactory with respect to the heat and solvent resistances. Hence, known polyimides have merits and demerits in performance.
The present inventors had proposed polyimides of recurring units of the following formulae (I) to (IV) which have good mechanical, thermal and electrical properties and good resistances to solvent and heat: ##STR1## (Japanese Patent Application No. 62-076095) ##STR2## (Japanese Patent Application Nos. 62-163940 and 62-163941) ##STR3## (Japanese Laid-open Patent Application No. 62-53388) ##STR4## (Japanese Laid-open Patent Application No. 62-50372).
In the above formulae, each X represents a carbonyl group or a sulfonyl group, and each R represents a tetravalent group selected from the group consisting of aliphatic groups having 2 or more carbon atoms, alicyclic groups, monocyclic aromatic groups, condensed polycyclic aromatic groups, and non-condensed polycyclic aromatic groups where aromatic groups are connected directly to each other or through crosslinkage.
These polyimides have good fluidity and good processability, but when they are kept at high temperatures over a long term (e.g. when they are retained over a long time at high temperatures in a cylinder at the time of injection molding), the fluidity of the resin melt gradually decreases with an attendant decrease in forming processability.
Moreover, the above polyimides may undergo melt forming at high temperatures, but there is a demand of the development of polyimides which exhibit stable fluidity when formed at lower temperatures.